The World Health Organization says that at least 135 people in China have been sickened by the H7N9 flu strain, and 44 have died. Most likely, these victims got the virus from chickens sold in live poultry markets. But where did the chickens get it? From ducks, who got it from wild migratory birds, scientists now say.

In fact, migratory birds that spent time in Hong Kong probably transferred the key N9 influenza genes to domestic ducks at least twice before the H7N9 outbreak began in March of this year, according to a report published Wednesday by the journal Nature.

In addition, wild migratory birds from Eurasia carried an H9N2 flu strain that contributed some genes to the H7N9 strain that went on to infect humans.

But the researchers emphasized the key role of ducks in bringing all the ingredients together to create H7N9.

"Domestic ducks seem to act as key intermediate hosts by acquiring and maintaining diverse influenza viruses from migratory birds, by facilitating the generation of different combinations of [...] viruses, and by transmitting these viruses to chickens," they wrote.

Influenza viruses are compact infectious agents controlled by only eight genes. They are known by their "H" (hemagglutinin) and "N" (neuraminidase) genes, which contain instructions for the two proteins that stud the exterior of the virus. Hemagglutinin allows flu particles to attach to host cells. After they get inside and replicate, neuraminidase allows the new flu particles to exit and find new cells to infect.

The hemagglutinin in the H7N9 strain has a mutation that has been shown to help the virus replicate in the upper respiratory tract of birds, the researchers reported. But the virus doesn't seem to have any mutations that make it particularly well-adapted to humans. Nothing in the genetic history of the virus suggests that a mammalian host was key to its evolution, they said.

As the scientists investigated the origin of the H7N9 flu virus, they also discovered a closely related H7N7 virus in chickens sold at live poultry markets in the Chinese province of Wenzhou. The N7 in this strain has been circulating in ducks since at least 2010, they determined.

Although H7N7 has never been detected in people, the researchers wanted to see whether it was capable of sickening mammals. So they infected a dozen ferrets -- animals often used as stand-ins for people in flu experiments because their symptoms are similar to ours -- and saw signs that the virus became established after two days. Over time, virus particles could be found in the noses, tracheas, lungs and lymph nodes of the animals.

The surprise discovery of H7N7 is a concrete sign that the threat of flu viruses from wild birds extends beyond H7N9, the researchers said.

"The current pandemic threat extends beyond the H7N9 virus," they wrote. "To control H7N9 and related viruses ultimately, it is necessary to reconsider the management of LPMs (live poultry markets) in urban areas."